Following the recent development of Internet Protocol (IP) networks, Ethernet (registered trademark) has been suggested to replace services that have been provided over a dedicated line (for example, inter-enterprise communication). However, unlike a dedicated line, Ethernet is based on a different concept and thus has no scheme to ensure high-quality communication.
This has not been a problem in conventional IP networks since the networks are mainly used for data communication. On the other hand, fluctuation (variation in transmission delay) does not occur in optical transport networks (OTNs) such as synchronous digital hierarchy (SDH) since the communication speed is based on n times the 64 Kbps for audio. As SDH is being replaced with Ethernet, implementation of dedicated line networks on Ethernet has come under consideration.
Thus, in the development of a means for replacing a transmission apparatus on a dedicated line requiring high-quality communication, it becomes a problem that fluctuation occurs in a long-haul transmission using Ethernet packets when the packets pass through multiple ring networks. For example, fluctuation occurs when the packet interval, which is constant at the transmission side, increases/decreases during transmission due to arbitration of signals from other systems. In audio communication such as Voice over IP (VoIP), for example, fluctuation becomes audio distortion.
For example, although Ethernet prioritizes the signals from the other systems and transmits packets of a high priority first, among packets that have arrived at the same time, fluctuation still occurs if the priority is the same. Further, a transmission apparatus of Ethernet has a function of operation administration maintenance (OAM), and regularly transmits OAM frames for inspection at the rate of, for example, 300 frames/second according to the ITU recommendation. These OAM frames that are regularly transmitted and inserted between packets are also one cause of fluctuation.
Fluctuation cannot be suppressed by prioritization since the OAM frames have a high priority and are more likely to be inserted between packets than other packets. Further, since stations through which the packets pass insert the OAM frames for respective purposes, the number of inserted OAM frames increases as the number of the stations increases, thereby increasing the fluctuation.
In SDH and asynchronous transfer mode (ATM), technology is known that minimizes delay of packets due to insertion of OAM frame (see, for example, Japanese Laid-Open Patent Publication Nos. H08-237254 and 2000-224175). In this technology, a null packet (null cell) that has a fixed length and is transmitted during an interval between packets is detected based on the header of each frame, and the OAM frame is inserted between the detected null packets.
However, the intervals between packets are indefinite in a communication network such as Ethernet where no null packet is transmitted during the intervals between packets. Thus, according to the conventional technology described above, the data length of a frame inserted between packets may be longer than the interval between packets. In this case, it becomes a problem that packets subsequent to the inserted frame are delayed, thereby increasing variation (fluctuation) in transmission delay.